I use a thin piece of plexiglass, slightly bigger than the print. I taped a piece of nice paper over it (larger than the print, but smaller than the plexiglass) to keep it in place. Worked nice...gives a bit of bend without a fold/crease. As obvious as it may sound, be sure to stamp fragile all over it too.

The tradeoff for rigidity is weight, but depending on the product, it might be worth it. I don't think the shipping went up too much in that case. Some smaller hardware stores will cut you custom plexiglass, and perhaps some custom sign shops would give you a deal on scrap.

Another option is giving it to a UPS store, many stores will offer a guarantee if they pack it, at no extra cost...at least when I did it. I used it only once and it was more than I anticipated, but the item was relatively heavy.